1. Field of the Invention
The present invention relates to a transmission apparatus and method for use in a mobile communication system based on an OFDM (Orthogonal Frequency Division Multiplexing) scheme, and more particularly to a transmission apparatus and method for supporting a multiple access scheme on the basis of the OFDM scheme.
2. Description of the Related Art
Recently, mobile communication systems have developed from mobile systems for providing a user with voice signals to a wireless packet communication system for providing the user with high-speed and high-quality wireless data packets, such that anyone can use a variety of data services and multimedia services. The third-generation mobile communication system is classified into an asynchronous 3GPP (3rd Generation Partnership Project), and a synchronous 3GPP2, each of which are being standardized for implementing high-speed and high-quality wireless packet communication services. For example, an HSDPA (High Speed Downlink Packet Access) standardization is in progress in the 3GPP, and a 1xEV-DV (1xEvolution Data and Voice) standardization is in progress in the 3GPP2. The 1xEV-DV standardization for the downlink is “IS-2000 Release C”. Presently, the standardization of the uplink is in progress, and the uplink is being standardized under the title “IS-2000 Release D”. The aforementioned standardizations are needed for users or subscribers to receive high-speed (more than 2Mbps) and high-quality wireless data packet transmission service in the third-generation mobile communication systems. A fourth-generation mobile communication system is needed for users or subscribers to receive higher-speed and higher-quality multimedia communication services.
Technical solutions for providing high-speed and high-quality wireless data packet services require improved software capable of developing more diverse and abundant contents and improved hardware capable of developing wireless access technology, which can provide users with excellent services and has high spectrum efficiency.
The aforementioned improved hardware, from among the technical solutions, will firstly be described.
The high-speed and high-quality data service available in wireless communication is generally deteriorated by channel environments. Channel environments for wireless communication are frequently changed by a variety of factors, for example, white noise, variation in reception signal power, shadowing, movement of a Subscriber Station (SS), a Doppler effect caused by a frequent change in speed, and interference caused by other users or a multi-path signal. Therefore, in order to provide the aforementioned high-speed wireless data packet service, technology improvements over the second-generation or third-generation mobile communication systems, and which improves adaptive capacity for channel variation, is required. Although a high-speed power control scheme for use in the conventional system enhances adaptive capacity for such channel variation, the 3GPP and the 3GPP2 which are currently processing a high-speed data packet transmission system standard commonly utilize an Adaptive Modulation and Coding (AMC) scheme and a Hybrid Automatic Repeat Request (HARQ) scheme.
The AMC scheme changes a modulation scheme and a code rate of a channel encoder according to a variation in the downlink channel. In this case, downlink channel quality information can generally be obtained by measuring a Signal-to-Noise Ratio (SNR) of a signal using the SS functioning as a terminal receiver. The SS transmits the channel quality information to a BS (Base Station) through an uplink. The BS predicts the downlink channel conditions on the basis of the downlink channel quality information, and prescribes an appropriate modulation scheme and a code rate for a channel encoder on the basis of the predicted downlink channel condition. Therefore, an Adaptive Modulation and Coding (AMC) system adapts a high-order modulation scheme and a high code rate to an SS (Subscriber Station) having an excellent channel. However, the AMC system adapts a low-order modulation scheme and a low code rate to an SS having a relatively inferior channel. Typically, the SS having the excellent channel may be positioned in the vicinity of the BS, and an SS having the relatively inferior channel may be positioned at a cell boundary. The aforementioned AMC scheme can largely reduce an interference signal as compared to a conventional scheme dependent on a high-speed power control, resulting in the improvement of system performance.
If an unexpected error occurs in an initial transmission data packet, the HARQ scheme requires retransmission of the data packet to compensate for the erroneous packet. In this case, a predetermined link control scheme is adapted to the compensation process of the erroneous packet, and is considered to be equal to the HARQ scheme. The HARQ scheme is called a Chase Combining (CC) scheme, and is classified into a Full Incremental Redundancy (FIR) scheme and a Partial Incremental Redundancy (PIR) scheme. The CC scheme transmits all of the packets during a retransmission operation in the same manner as in an initial transmission process. In this case, a reception end combines the retransmission packet with the initial transmission packet to increase reliability of a coded bit received in a decoder, such that the reception end can obtain an overall system performance gain. In this case, if two equal packets are combined with each other, an effect similar to that of an iterative coding occurs, and a performance gain of about 3 dB can be attained on average.
The FIR scheme retransmits a packet composed of only redundant bits generated from a channel encoder, instead of retransmitting the same packet, such that the FIR scheme can improve a coding gain of a decoder included in the reception end. That is, the decoder uses both initial transmission information and new redundant bits during a decoding process, resulting in an increased coding rate. As a result, the decoder's performance can also be improved. It is well known in coding theory that a performance gain obtained by a low coding rate is greater than a performance gain obtained by an iterative coding rate. Therefore, when considering only the performance gain, the FIR scheme provides better performance than that of the CC scheme.
The PIR scheme transmits a data packet which is composed of a combination of information bits and new information bits during a retransmission time. The PIR scheme combines the information bits with the initial transmission information bits during a decoding process, such that it can obtain an effect similar to that of the CC scheme. Also, the PIR scheme performs the decoding process using the redundant bits, such that it can obtain effects similar to those of the FIR scheme. The PIR scheme has a coding rate greater than that of the FIR scheme, such that it's the PIR scheme performance is considered to be intermediate between individual performances of the FIR and CC schemes. However, the HARQ scheme must consider a variety of factors related to performance and system complexity (e.g., a buffer size and a signaling, etc.), such that it is difficult to determine either one of the factors.
Provided that the AMC scheme and the HARQ scheme use independent techniques capable of improving the adaptive capacity for a channel variation or are combined with each other, system performance can be greatly improved. In other words, if a modulation scheme suitable for the downlink channel conditions and a coding rate of the channel encoder are determined by the AMC scheme, a data packet corresponding to the determined information is transmitted.
However, although the aforementioned two schemes are used, the principal problem encountered in radio communication, i.e., the lack of radio resources, is incapable of being solved. In more detail, multiple access—based technology having a superior spectrum efficiency must be developed to maximize subscriber capacity and to enable high-speed data transmission requisite for a multimedia service. In conclusion, a new multiple access scheme having superior spectrum efficiency must be developed to provide the high-speed and high-quality packet data service.